Chemical Energetics: Exothermic and Endothermic Reactions

Vocabulary practice flashcards covering chemical energetics, exothermic and endothermic reactions, reaction profiles, enthalpy changes, and bond energy calculations.

The System

The chemicals reacting in a chemical process.

The Surroundings

Everything not included in the chemicals reacting.

Chemical Bonds

Tiny stores of chemical energy that are broken and formed during a chemical reaction.

Exothermic Reaction

A reaction in which thermal energy is transferred from the system to the surroundings, leading to an increase in the temperature of the surroundings.

Endothermic Reaction

A reaction in which thermal energy is taken in from the surroundings to the system, leading to a decrease in the temperature of the surroundings.

Examples of Exothermic Reactions

Combustion, neutralisation, rusting, and reacting a metal with water.

Examples of Endothermic Reactions

Electrolysis, thermal decomposition, and photosynthesis.

Applications of Exothermic Reactions

Heat packs for mountain climbers, hand warmers used in winter, and self-heating cans for food and drinks such as coffee and hot chocolate.

Applications of Endothermic Reactions

Cold packs used in sports injury treatment to take heat away from a recently injured area to prevent swelling.

Enthalpy ($H$)

A measure of the total heat of reaction of a chemical reaction.

Enthalpy Change ($\Delta H$)

The transfer of thermal energy during a reaction, which is negative for exothermic reactions and positive for endothermic reactions.

Activation Energy ($E_a$)

The minimum energy that colliding particles must have to react.

Reaction Pathway Diagrams

Graphical representations of the relative energies of reactants and products in chemical reactions, with energy on the y-axis and the reaction pathway on the x-axis.

Overall Energy Change (Reaction Diagram)

The difference in height between the energy of the reactants and the energy of the products on a reaction pathway diagram.

Reaction Pathway Diagram Arrows

Directional indicators showing whether a reaction is exothermic (downwards pointing arrow, indicating lost energy) or endothermic (upwards pointing arrow, indicating gained energy).

Bond Breaking

An endothermic process that always requires energy to be taken in from the surroundings to break chemical bonds.

Bond Making

An exothermic process in which energy is transferred to the surroundings as new chemical bonds form.

Overall Exothermic Reaction (Bond Energies)

A reaction where more energy is released when new bonds are formed than is required to break bonds in the reactants, yielding a negative $\Delta H$ value.

Overall Endothermic Reaction (Bond Energies)

A reaction where more energy is absorbed to break bonds than is released to form new bonds, yielding a positive $\Delta H$ value.

Bond Energy

The specific amount of energy required to break a bond or the amount of energy given out when the bond is formed, measured in $kJ\,mol^{-1}$.

Energy Change Equation

$\text{Energy change} = \text{Energy taken in} - \text{Energy given out}$ (or $\text{energy in} - \text{energy out}$).

Energy In

The sum of all the bond energies for all the bonds broken in the reactants.

Energy Out

The sum of all the bond energies for all the bonds formed in the products.

Energy Change of $H_2 + Cl_2 \rightarrow 2HCl$

An exothermic reaction where energy in is $679\,kJ\,mol^{-1}$ and energy out is $864\,kJ\,mol^{-1}$, giving an overall energy change of $-185\,kJ\,mol^{-1}$.

Energy Change of $2HBr \rightarrow H_2 + Br_2$

An endothermic reaction where energy in is $732\,kJ\,mol^{-1}$ and energy out is $629\,kJ\,mol^{-1}$, giving an overall energy change of $+103\,kJ\,mol^{-1}$.

Combustion of Hydrogen ($2H_2 + O_2 \rightarrow 2H_2O$)

An exothermic process releasing energy with an overall change calculated as $2 \times 436 + 498 - 4 \times 464 = -486\,kJ\,mol^{-1}$.

Combustion of Methane ($CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$)

An exothermic reaction shown on an energy level diagram with products lower than reactants, having an energy change of $-891\,kJ\,mol^{-1}$.

Nitrogen Trifluoride ($NF_3$)

A compound essential to the electronics industry, produced by reacting ammonia ($NH_3$) with fluorine ($F_2$).

Exothermic Reaction Heat Flow Diagram

A diagram illustrating a reaction mixture that is hotter than its surroundings, releasing heat outward into the surroundings.

Endothermic Reaction Heat Flow Diagram

A diagram illustrating a reaction mixture that is cooler than its surroundings, absorbing heat inward from the surroundings.